Protocol Buffers In C#: How Are Boxed Value Types Handled

Question

In the following examples:

public class RowData
{
    public object[] Values;
}

public class FieldData
{
    public object Value;
}

I am curious as how either protobuf-net or dotnet-protobufs would handle such classes. I am more familiar with protobuf-net, so what I actually have is:

[ProtoContract]
public class RowData
{
    [ProtoMember(1)]
    public object[] Values;
}
[ProtoContract]
public class FieldData
{
    [ProtoMember(1)]
    public object Value;
}

However I get an error saying "No suitable Default Object encoding found". Is there an easy way to treat these classes, that I am just not aware of?

To elaborate more on the use case:

This is a scaled down version of a data class used in remoting. So essentially it looks like this:

FieldData data = new FieldData();
data.Value = 8;

remoteObject.DoSomething(data);

Note: I've omitted the ISerializable implementation for simplicity, but it is as you'd expect.

Answer 1

Re protobuf-net, which I maintain:

The issue here isn't value-types (which it will often handle fine) - it is the open object usage, which means it simply doesn't know what data to expect, and thus how to encode/decode it.

At the moment, I can't think of an easy/clean way to handle that. It will handle a range of common value-type scenarios, lists, and any level of hierarchy based on contracts (data-contract, proto-contracts, or some xml-schemas), but it needs a clue.

Perhaps if you can clarify the use-case, I might be able to help more? For example, the above wouldn't work very with DataContractSerializer or XmlSerializer either...

Re dotnet-protobufs; I can't really comment, but I'm pretty sure it would be even less forgiving; it is intended to be used with classes generated from a .proto file, so object would simply never enter into the model (Jon: correct me if I am wrong).

If you do leave more info, could you kindly post a comment here? So I can find it easily... Alternatively, drop me a mail directly (see my SO profile).

---

edit - here's the hacky thing I had in mind - it isn't working at the moment, but I'll figure out why tomorrow (probably). Note that in theory the extra members could all be private - I'm just trying to make it easy while debugging. Note that this doesn't take any extra storage. Like I say, it doesn't work today, but it should - I'll find out why...

[ProtoContract]
public class FieldData
{
    public object Value {get;set;}

    [ProtoMember(1)]
    public int ValueInt32 {
        get { return (int)Value; } set { Value = value; } }
    public bool ValueInt32Specified {
        get { return Value != null && Value is int; } set { } }

    [ProtoMember(2)]
    public float ValueSingle {
        get { return (float)Value; } set { Value = value; } }
    public bool ValueSingleSpecified {
        get { return Value != null && Value is float; } set { } }

    // etc for expected types
}

Answer 2

This is something like what I had in mind. Let me know what you think. Naturally I'd have to add a subclass for each value type I need to support. What do you think? Is there a better way, do you see any inefficiencies with this method?

[ProtoContract, Serializable]
[ProtoInclude(1, typeof(Int32FieldData))]
public abstract class FieldDataBase : ISerializable
{
 [ProtoIgnore]
 public abstract object Value { get; set;}
 protected FieldDataBase()
 { }

 #region ISerializable Members
 protected FieldDataBase(SerializationInfo info, StreamingContext context)
 {
  Serializer.Merge<FieldDataBase>(info, this);
 }
 public void GetObjectData(SerializationInfo info, StreamingContext context)
 {
  Serializer.Serialize<FieldDataBase>(info, this);
 }

 #endregion
}

[ProtoContract, Serializable]
public class Int32FieldData : FieldDataBase
{
 [ProtoMember(1)]
 public int? Int32Value;

 [ProtoIgnore]
 public override object Value
 {
  get { return this.Int32Value.HasValue ? this.Int32Value : null; }
  set { this.Int32Value = (int?)value; }
 }
 public Int32FieldData() { }
 protected Int32FieldData(SerializationInfo info, StreamingContext context)
  :base(info, context)
 { }
}

Answer 3

Direct encapsulation seems to work fine with no additional overhead from all the properties, in the following manner:

[ProtoContract, Serializable]
public class ObjectWrapper : ISerializable
{
 public ObjectWrapper()
 { }
 [ProtoIgnore]
 public object Value
 {
  get
  {
   if (Int32Value.HasValue)
    return Int32Value.Value;
   else if (BinaryValue != null)
    return BinaryValue;
   else
    return StringValue;
  }
  set
  {
   if (value is int)
    this.Int32Value = (int)value;
   else if (value is byte[])
    this.BinaryValue = (byte[])value;
   else if (value is string)
    this.StringValue = (string)value;
  }
 }
 [ProtoMember(1)]
 private int? Int32Value;
 [ProtoMember(2)]
 private string StringValue;
 [ProtoMember(3)]
 private byte[] BinaryValue;
            // etc

 #region ISerializable Members
 protected ObjectWrapper(SerializationInfo info, StreamingContext context)
 {
  Serializer.Merge<ObjectWrapper>(info, this);
 }
 public void GetObjectData(SerializationInfo info, StreamingContext context)
 {
  Serializer.Serialize<ObjectWrapper>(info, this);
 }

 #endregion
}

Answer 4

(updated)

Right; figured it out... the main problem in my sample above was the value-getters; they were throwing exceptions. There were also some library glitches (now fixed).

However, the simplest approach is Nullable<T> pass-thru properties:

    [ProtoMember(1)]
    private int? ValueInt32
    {
        get { return Get<int>(); }
        set { Value = value; }
    }

etc, with:

    private T? Get<T>() where T : struct
    {
        return (Value != null && Value is T) ? (T?)Value : (T?)null;
    }

Both this and the *Specified approach have been tested, and now work fine.